find attr filepath correctly

The attribute filepath is easier now that I can walk a fs_tree backwards
and get a path back.

libsaria/collection/library.py

@@ -89,8 +89,7 @@ def get_attrs(id, *attrs):
 		if attr == "id":
 			res += [id]
 		elif attr == "filepath":
-			cmp = rec.fs.walk_backwards()
-			res += [libsaria.path.sep.join([""] + cmp)]
+			res += [rec.fs.walk_path_backwards()]
 		elif attr == "art":
 			from libsaria import lastfm
 			res += [lastfm.get_artwork_id(id)]